Polyoxyalkylene glycol derivatives are used as lubricating oils in various applications. For example, water-soluble polyoxyalkylene glycol derivatives are used as hydraulic oils, cutting oil, grinding oil, gear oil, etc., polymers of polyoxyalkylene glycol (meth)acrylates are used as water-soluble lubricants (JP-A-56-47411), and compounds obtained by esterifying alkylene oxide adducts of castor oil are used as metal-working oils (JP-A-57-207699), while oil-soluble polyoxyalkylene glycol derivatives are used as viscosity index improvers for mineral oils (JP-B-53-44196), a refrigerating machine oil (JP-A-57-51795), etc. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
However, polymers of polyoxyalkylene glycol (meth)acrylates and derivatives of castor oil are defective in that their performances change if they are used in the form of an aqueous solution for a prolonged period of time, while oil-soluble polyoxyalkylene glycols also have the problem that their applications are limited since not only are they quite insoluble in water, although quite soluble in mineral oils, but it is impossible to emulsify mineral oils or the like in water.